This invention relates to powder metallurgy and, more particularly, to a method for substantially eliminating the precipitation of carbides at prior powder particle boundaries in articles made by powder metallurgy.
The undesired formation of carbides at prior particle boundaries occurs during powder metallurgy processes after the powdered alloy has been heated above the carbide solvus temperature. When the microstructure of the cooled alloy is examined, the prior particle boundaries can be seen distinctly delineated. And upon further examination, it has been determined that the materials which delineate boundaries are mainly reprecipitated carbides. Here and throughout this application it is intended by the term "carbides" to include compounds of carbon with one or more metals with or without one or more nonmetals such as oxygen, sulfur and nitrogen.
It is not completely understood why such carbides form at the prior particle boundaries. When the alloy is heated above the carbide solvus temperature, the carbides within the powder particles go into solid solution. When the temperature is then lowered below the carbide solvus temperature, carbides reprecipitate. While at elevated temperature, the powder particles are ideally desired to fuse together to form a solid, unitary article without the particles themselves melting. When the carbides precipitate along what had been the particle boundaries rather than remaining more homogeneously distributed within the particles, the resulting article does not have the desired properties, as evidenced most notably by low ductility in directions perpendicular to the delineated prior boundaries. Furthermore, it has been found that alloys which are vulnerable to sulfidation attack, which is particularly undesirable in products such as components of engines which burn fuels containing sulfur are improved by the present process.
One method for solving the problem of boundary delineation is proposed in Allen U.S. Pat. No. 3,890,816, June 24, 1975, relating to the elimination of carbide segregation to prior particle boundaries in nickel-base alloys by means of adding a strong MC-type carbide former selected from the group consisting of columbium, tantalum, hafnium and zirconium to the alloy melt. The added carbide formers seem to prevent boundary delineation by strongly binding the carbon within the particles, thus preventing carbide precipitation at the boundaries. However, this requires adding a significant amount of such carbide formers, which substantially alters the composition of the treated alloys and which in turn may significantly affect the alloys' properties or may result in different properties, so that the modified composition may not receive the same acceptance in industry as the unmodified alloys. In the case of superalloys for aircraft engines this is a particularly serious problem, since under existing regulations concerning qualification testing, such modified alloys cannot be used in many of their intended applications without extensive and costly testing to determine the properties of articles made from the new alloy. FNT The same solution was subsequently published in Metals Engineering Quarterly, Nov. 1974, pages 47-49.